Navigation system

ABSTRACT

A navigation system including at least one pulse radar and a number of transponders which when receiving radar pulses transmit high frequency pulse modulated signals. When receiving an interrogating signal from the pulse radar the transponder transmits a response signal with a delay. The delay is selected in such a manner that the response signal arrives at the pulse radar within a time space lying between the moment when all signal echos, lying within a definite range, have been received and the moment when the next radar pulse begins. The display scan in the radar is delayed by the same value as the response signal. In this way the response signal competes with considerably weaker echo signals than in the case of a non delayed response.

FIELD OF INVENTION

The present invention relates to a navigation system including at leastone vessel-borne radar arrangement and at least one radar beacon whichwhen receiving radar pulses transmits high frequency, pulse modulatedresponse signals.

BACKGROUND

In most of known navigation systems of the above said type, the radarbeacon responds at a frequency which is near the frequency of thereceived radar pulse and the response signal is presented on a displaysuperposed on the normal radar image. An important problem is thatpowerful ground return, rain- and sea clutter can completely orpartially mask the signal of the transponder particularly if thetransponder is situated a short distance away. The fact that the signalof the radar beacon is superposed on the normal radar display may alsoallow the response signal hide other echos of interest which is alsoconsidered as a drawback. For civil-maritime radar communication thereare two allotted frequency bands, the so-called 3 cm. band or X-bandbetween 9300 and 9500 MHz and the so-called 10 cm. band or S-bandbetween 2900 and 3100 MHz.

In the case of so-called permanent frequency radar beacons, thesetransmit, when detecting a radar pulse, a response pulse with apermanent frequency, for example, in the lower part of said frequencyband. The vessel-borne radar can be readjusted for receiving such signaland in this way there is made an attempt to distinguish the responsesignal from disturbing signals possibly arising in consequence of thetransmitted radar signal. However, in a vessel-borne radar withfrequencies near the response frequency, disturbing signals also ariseat the response frequency in the form of ground- and sea clutter. Thereason is that the transmitted radar signal and consequently the clutteris broad in frequency and thus the clutter will be situated within thefrequency range of the transponder. Furthermore, the radar receiver hasa finite attenuation for the clutter frequency even when the receiver isadjusted to the response frequency. For these radars also an effectivereceiver selection is necessary in order to prevent racon signals atfrom resulting in interference on the normal radar display. Anotherdrawback, is when the radar and the radar beacon are transmitting withdifferent frequencies, is that the majority of radar aerials existingtoday are of the "end fed slotted waveguide" type which has thecharacteristics that the direction of the aerial beam is frequencydependent and varies at 0.8°-1.0°/100 MHz. This can result in adeviation of bearing to the radar beacon and furthermore in a stronglyreduced range of the same.

Another approach is the so-called frequency offset method in which theresponse transmitter answers with a definite frequency which is forexample 50 MHz below the frequency transmitted from the radar. Also inthis case the interference signal arises for the same reasons asmentioned above in connection with permanent frequency radar beacons.Reference is made, for example, Conceptual Radar Piloting TechniquesUsing Radar Beacon (Racon) Technology and Other Advanced Marine RadarTechnology, E. F. Greneker and J. E. Metthews, Georgia Institute ofTechnology, April 1981 and Fixed Frequency Racon, a performance analysisprepared for the Swedish Administration of Shipping and Navigation, byBo Morwing and P. O. Gustavsson.

SUMMARY OF THE INVENTION

An object of the present invention is to obtain a clutter free receivingof response signals of the above indicated type and this is achieved dueto the fact that receiving is carried out when clutter occurs in aminimal extension. It is a further object to allow the transponderaccording to the invention to function together with existingconventional vessel-borne radar equipments of arbitrary type as well aswith vessel-borne radar equipment which by means of code signals canreadjust the transponder to the transmitting mode according to theinvention.

BRIEF DESCRIPTION OF DRAWINGS

The invention will next be described with reference to the attacheddrawing in which

FIG. 1 shows the display of a vessel-borne radar during conventionalreception;

FIG. 2 is a frequency diagram showing the spectrum of the vessel-borneradar and of the response signal in case of permanent frequency answerand displaced frequency answer respectively;

FIG. 3 shows the display of a vessel-borne radar according to theinvention;

FIG. 4 is a block diagram of a transponder according to the invention;

FIG. 5 is a block diagram of a vessel-borne radar according to theinvention; and

FIG. 6 is a range diagram showing the position of the response signalwithout using and with using the invention.

DETAILED DESCRIPTION

FIG. 1 shows diagrammatically the display of a vessel-borne radar withresponse signals from two transponders A and B which are coded withMorse-Code signals w and k respectively. By C is designated rainreflections or so called rain clutter and by D is designated a stretchof coast line which causes strong ground return on the display. As aconsequence, the response signal from the responder B will be difficultto distinguish due to the rain clutter and part of the response signalwill be drowned by the ground return from land.

The known methods for eliminating such drawback are based on theprinciple that the response signal has another frequency than thefrequency transmitted from the vessel-borne radar. They give, however,no satisfying solution for distinguishing the response signals and thereceived echos.

FIG. 2 shows a frequency spectrum in the so-called X-band between 9300and 9500 MHz used for civil-maritime radar traffic where x=9300-9320 MHzis disposed for permanent frequency racons. According to the permanentfrequency principle, the responder, when receiving the signal of thevessel radar, sends a response pulse with a definite frequency in thelower part of the band for example 9310 MHz. If the frequency of thevessel-borne radar is near this frequency for example 9350 MHz, adisturbing clutter can arise which can be of the same order of magnitudeas the response signal and which masks the signal of the transponder asshown in FIG. 2. According to the so-called frequency offset principlethe transponder sends its signal with a frequency which with a certainvalue, for example 50 MHz, is below the frequency of the vessel-borneradar which is for example 9465 MHz. In this case, it is also notpossible to avoid clutter and masking of the signal of the transponderas appears from FIG. 2. Furthermore, in both these cases, a change inthe existing radar equipment will be necessary if these are built onlyfor receiving their own frequencies.

A fundamental idea of the invention consists in that the transponder,which can be of arbitrary type, (ie., can transmit with the receivedfrequency, with a permanent frequency or with a frequency offsetrelatively to the received frequency) transmits its signal with a delay.The delay should be so large that the response signal arrives at thevessel radar at first when all echos within a definite range havearrived. In this way, the response signal only needs to complete withweaker echos and the risk of masking by clutter and other echos isstrongly reduced.

This appears from FIG. 3 which shows the same display as FIG. 1 with thedifference that the transponder B in FIG. 1 has been equipped forsending a delayed response signal and the transmitter has been switchedover to the transmitting mode according to the invention, whereby thescan is started with a delay. The signal from the transponder A will notbe presented as it is of the conventional type and its response signalhas been received before the display scan has started. The same concernsthe rain clutter and the coast countour line D in FIG. 1. The signalfrom the transponder B which is delayed for a certain time will,however, be presented at the correct range from the display as the startof the display scan has been delayed during the same period of time asthe signal of the transponder. As appears from FIG. 3, the responsesignal has been coded with an additional Morse-Code signal s (. . . )which indicates that the response is coming from a transponder indelayed mode. G designates weak echos at a range corresponding to thedelay range. These echos are thus presented at a false distance but thisis without interest as the only purpose is to make it possible todistinguish the racon response at a correct range.

FIG. 4 shows diagramatically in a block diagram a transponder of theconventional type to which the idea of the invention has been applied.By 1 is designated an aerial, by 2 a transmitter/receiver switch and by3 a receiver from which the received signals are supplied to atransmitter 4 through a high frequency generator 5. A modulator 6 isarranged for modulating the high frequency signal of the transmitter andthe modulator is controlled by a synchronizing signal generator 7activated by the received pulse signal.

Transponders of the above type are generally known and are described forexample in Swedish Pat. No. 419,002. According to the invention, thetransponder transmits a delayed response signal when identifying aninterrogating signal from a vessel-borne radar which is equipped forreceiving such a delayed response signal. By 8 is designated anidentifier for interrogating signals which when receiving theinterrogating signal affects through a control device 11 a symbolicallyindicated switch 9 for connecting a delay circuit 10 between thesynchronizing signal, generator 7 and the modulator 6. By delaying theresponse signal the latter will be interpreted by the vessel-borne radaras if it should come from a greater distance than it comes from inreality. In order to prevent that the vessel-borne radar reproduces theresponse signal at an incorrect distance, it is provided with the samedelay as the transponder so that the image appears at a correct distancewhile all other echos within a definite range disappear.

When for example the pulse repetition frequency is 1 KHz and thetransponder and the radar scan are delayed by 0.5×1 PRF=0.5 ms., theanswer from the responder will compete with radar echos lying furtherthan

    (0.5×10.sup.-3 /2)×3×10.sup.-9 =75 Km.

which when having a distance of 1 Km. or less between transponder andvessel radar gives a relative improvement of the "tolerance againstclutter" of the order of magnitude 75 dB.

In order to allow co-operation with a vessel radar which is not equippedwith delay arrangements, the switch 9 in its other position shunts thedelay circuit 10. FIG. 15 shows a conventional pulse radar including asynchronizing signal generator 20 which starts a modulator 21 formodulating the high frequency signals of a magnetron 22. The pulsemodulated signals are supplied to an aerial 24 through atransmitter/receiver switch 23. On reception the target echo signalcomes through the switch 23 to an HF amplifier 25 the output of which issupplied to a mixer 26 together with the signal from a local oscillator27 and from the mixer the signal is supplied to an MF amplifier 28. Thenthe signal comes to a detector 29 and through a video amplifier 30 tothe display 31. The synchronizing signal generator 20 activates a scangenerator 32 which generates the scan of the display 31. It is alsopossible to let the transmission pulse trigger the scan generatorindirectly. For making possible that this conventional device can workwith a delay which corresponds to the delay of the transponder, a delaycircuit 33 is connected which delays the scan signal with the same valueas in the transponder. In order to switch over the transponder totransmission of the delayed response signal, the vessel-borne radar hasto send an interrogating signal which for example can consist of a pulseseries of special shape or a double pulse. When supposing that theinterrogating signal consists of for example an extended pulse, this isrepresented by a switch 35 which when operating causes a special lengthof the modulating pulse and at the same time switches over a switch 34in consequence of which the direct connection between the synchronizingsignal generator 20 and the scan generator 32 is interrupted and thedelay circuit 33 is connected between these generators. In consequenceof this, only the signal from the transponder B will be represented onthe display together with remote echos and the response signal from thetransponder. A will be received before the scan of the display isstarted as mentioned in connection with FIG. 3.

In order to simplify the description, a vessel-borne radar has beenchosen as an example and also transponders of the most simple form,where the response signal has the same frequency as the signal receivedfrom the vessel-borne radar. It is possible of course to apply theinvention to arrangements of other types for example sweep frequencyracon, permanent frequency racon or frequency offset racon.

It is possible that the transponder sends delayed signals only whenreceiving interrogating signals from the transmitter while otherwise itdoes not transmit any signals at all. This can be represented by aposition of the switch 9 in which it is not connected with any contact.

On the other hand it is possible that both the undelayed and the delayedresponse signal are transmitted after each other and only the delayedsignal is coded with a special character.

The alternatives above mentioned can be symbolized by the switch 9 whichcan connect the output of the synchronizing signal generator 7 todifferent contacts a-d. When contact a is connected, no signal istransmitted, when contact b is connected a normal response signal istransmitted which has no delay, when contact c is connected the responsesignal is transmitted delayed and when contact d is connected both theundelayed and the delayed signals are transmitted. Switching betweenthese positions can be carried out by means of interrogating signalswhich correspond to the different positions and have for exampledifferent pulse frequencies, or also manually during the installationwhen known in which mode the equipment will work. FIG. 6 is a rangediagram for a vessel-borne radar according to the invention. If the timespace between two transmitted pulses is 1 ms. (1 KHz.PRF) thiscorresponds to a detectable range of 150 Km. As shown in the diagram,the echo signal decreases with distance but having a response signal S1at a distance of for example 50 Km. this signal can be completely maskedby the echo signals. If on the other hand the response signal is delayedso that it occurs in a moment corresponding to a distance 75 Km. largerS2, it will compete with considerably weaker echo signals. FIGS. 6b and6c show the display scan in normal function and when delaying the vesselradar. It appears that with a delay of 0.5 ms. the contrast between theresponse signal and the disturbing echos increases considerably at thesame time as the response signal is represented at a correct range.

What is claimed is:
 1. A navigation system comprising at least one pulseradar means for transmitting interrogating radar pulses of determinablerepetition period and including a display means for making a display ofecho signals which may include reflected signals, and at least onetransponder means for receiving radar pulses and for transmitting inresponse to said pulses high frequency pulse modulated response signals,the transponder means including delay means and means associated withdelay means for transmitting, on receiving an interrogating signal fromthe pulse radar means, response signals with a delay such that responsesignals from the said transponder means are displayed on the displaywith displacement relative to the position that the response signalswould be displayed without said delay, said radar means including meansfor generating a scan on said display means and delay means for delayingthe start of the scan by the same value as the delay of the responsesignal whereby the echo signals which are received from objects within apredetermined range of said pulse radar means are received before thestart of the scan and thus are not shown whereas response signals fromthe transponder are displayed in the display, both said delay meansproviding respective delays of equal value and of such substantialfraction of the repetition period as to eliminate all but the strongestecho signals received after the start of the scan.
 2. A navigationsystem according to claim 1 wherein the transponder means includesswitching means which in response to an interrogating signal switchesover the transponder means from a rest condition to a working conditionin which a delayed response signal is transmitted.
 3. A navigationsystem according to claim 2, wherein the transponder means includessending means which in rest condition respond to a received radar pulsewithout said delay.
 4. A navigation system according to claim 2, whereinthe transponder means includes sending means which in rest condition donot transmit any signals.
 5. A navigation system according to claim 2,wherein the switching means includes means controlling the transpondermeans to transmit undelayed as well as delayed response signals.
 6. Anavigation system according to claim 1, wherein the radar andtransponder means includes means such that the said delay corresponds topart of the time space betwen two sequentially transmitted radar pulses.7. A navigation system according to claim 6, wherein the latter saidmeans are such that the said delay corresponds to about half of the timespace between two radar pulses.
 8. A navigation system according toclaim 2, comprising control means for setting the switching means torespond to a received interrogating signal.
 9. A navigation systemaccording to claim 1 wherein said switching means includes means for themanual adjustment of the switching means.